This invention relates to a method of making a threaded retainer ring for retaining a roller cutter on a drill bit, and more particularly to a method of making a retainer ring having external screw threads about its outer circumference for threaded engagement with a roller cutter to retain the roller cutter on a journal of the drill bit.
Heretofore, such as shown in U.S. Pat. No. 3,971,600 dated July 27, 1976, a roller cutter on a rotary drill bit has been mounted on a journal of the drill bit for rotation and retained on the journal by an externally threaded split ring. The split ring includes two semicircular portions which are first fitted within a groove in the journal and then the roller cutter is threaded onto the ring with the ring forming a thrust bearing.
It is necessary for a prolonged life of a cutter under adverse operating conditions that a proper fit occurs between the screw threads of the ring and cutter, and that minimum axial looseness occurs between the semicircular ring portions and the journal. If the screw threads are formed on the ring prior to the cutting of the ring into two portions, the two cuts made in the ring cause the ring halves to be out-of-round and undersized at right angles to the direction of the split.
It is necessary in order to obtain a relatively tight fit between the threads on the roller cutter and ring that minimal clearance be provided between the interfitting threads. If there is not adequate contact between the interfitting threads, possible mechanical wear and/or breakage problems such as stripping of threads may occur, particularly upon prolonged periods of use in certain types of hard formations. If the cut in a ring is around 0.003 inch or greater, an undesirable looseness may be provided between the roller cutter and ring resulting from the decreased outer diameter of the ring. While cutting methods utilizing lasers or thin diamond saws may provide cuts having a thickness less than 0.003 inch, such methods are expensive and/or time consuming. The present invention is particularly directed to cuts in a ring having a thickness between 0.003 inch and 0.045 inch.
Thus, it has been common heretofore to form the ring with *screw threads after the ring halves or semicircular portions have been formed. One method heretofore for providing screw threads on the separate semicircular portions of the ring has been to split lengthwise a tubular bar and then clamp or weld the two split portions together. Next, the outer peripheral surfaces of the split portions are finish machined and screw threads formed thereon. Then the inner periphery of the ring portions is finish machined. A plurality of semicircular ring portions may then be sliced from the length of the split tubular bar. Such a method for making the semicircular ring halves or portions is time consuming and expensive as a result of the several steps involved and the required machining tolerances.